In many chemical vessels it is necessary to provide mixing by the use of an axial rotating agitator. An example of such a vessel is a reactor used in producing polymer from monomer or in making pigments. In such vessels, it is desirable to wipe or scrape the internal wall of the vessel to minimize solids buildup and to increase heat transfer through the wall.
Experience has shown that wiping the interior of the wall using full length axially oriented wiper, has adverse effects on the effectiveness of the mixing of the contents of the vessel. Use of axially oriented wiper blades interferes with efficient agitation and causes temperature differentials within the contents of the vessel, which interfere with uniform processing and hinder attaining optimum conversion rates. In the design of a tubular vessel for optimum mixing and wall scraping, it is desirable to provide a helical agitator which rotates internally about the vessel axis. Such an arrangement is shown in Welch U.S. Pat. No. 1,966,325 and Barnese U.S. Pat. No. 3,194,385, which depicts helical conveyor screws having edges which wipe the interior walls of the vessel. Massey U.S. Pat. No. 301,506 employs spaced rigid cutters along a helical conveyor screw, while Coppage U.S. Pat. No. 1,028,934 forms the helix of rigid overlapping segments of his conveyor screw.
There are practical limitations on the design and construction of wipers on a helical agitator which act to limit transfer of the conveyor screw systems of the above-discussed patents into a wiper-agitator system. One limitation is that the wiper configuration needs not only to be helical, to match the helical agitator, but needs also to be flexible. A fixed helical blade possesses such a degree of stiffness that it cannot flex readily to contact the wall at all portions thereof. This stiffness prevents controlled deflection against the vessel wall so that a specific wiper-to-wall contact force cannot be obtained, and often results in wear or other damage to the vessel interior surface. In addition, controlled deflection wipers that deviate greatly from the helical configuration interfere with efficiency of the mixing action.
More specifically, the present invention is described as follows:
In a tubular vessel, an agitator to mix the contents of the vessel and to clean the wall of the vessel, which agitator comprises
(1) a shaft axially and centrally positioned in the vessel, which shaft is connectable to an outside rotational drive source,
(2) at least one spiral blade positioned helically around the shaft and supported away from the shaft by rigid supporting arms attached to the blade and the shaft, such blade proportioned in length and diameter to impel the contents in predetermined motion,
(3) a series of straight plate segments spaced at predetermined uniform intervals along the blade and attached to the blade such that the segments extend from the blade toward the interior wall of the tubular vessel, the plane of each plate relative to the axis of rotation being approximately the same angle as the helix angle of the periphery of the blade, but the plane of the plate relative to the helical blade being positioned to slant either away from or toward the direction of rotation of the shaft, said plate being constructed to flex with predetermined controlled pressure at the interior wall, and
(4) a flexible tip attached to the end of each segment and positioned and contoured such that by overlap of each tip with respect to the adjacent tip ends, all portions of the interior wall of the tubular vessel are contacted during a single revolution of the shaft.